Use an rtv like Permatex for water pumps and the T-Stat as a dressing for the gasket (you don't need gobs). This will fill in any surface imperfections and let it sit for 24hrs before filling. (or if you use Right Stuff I guess you can fill right away..but I prefer to use the Permatex rtv that's made made for pumps and stats).. Plus, I always tighten one side a little then do the other a little, then back and forth..wash rinse repeat until fully tightened then check the torque.
And as others suggested... clean the surfaces really good so that rtv can fill gaps and set well, taking care not to gouge anything and use a shop towel with brake cleaner before putting it all back together.